1. Field of the Invention
This invention relates to a measurement of fluid flows, and more particularly to a composite-type flow meter and a measuring method therefor which is adapted to correct a flow rate sensed by a thermal flow sensor by means of a correcting flow meter such as a Karman vortex flow meter.
2. Description of the Prior Art
There have been employed a variety of sensors for measuring flow rates of gaseous fluids. One of these sensors is a thermal flow sensor which is typically adapted to heat a resistive sensor element by feeding a current thereto and measure a flow of a gas, making use of the fact that the sensor element is cooled by the flow of the gas to cause changes in the resistance of the sensor element.
Recently, flow sensors of a type manufactured by the semiconductor technology, or so-called micro flow sensors have been known. The micro flow sensor is advantageous over the conventional thermal flow sensor in a remarkably fast response, a high sensitivity, a low power consumption, the adaptability to a mass production and so on.
The thermal flow sensor is generally disposed in a circular or rectangular conduit constituting a flow meter to measure a gas flowing through the conduit.
The conventional thermal flow meter, however, generates an output related to a mass flow, so that for a measurement of a volume flow the composition of gases under measurement must have been previously determined. Therefore, if the composition of gases changes in course of measurement, a correct value cannot be measured. In addition, the conventional thermal flow meter implies a problem that its output drifts due to aging changes during a long term of service.
Meanwhile, there has been known a Karman vortex flow meter as a highly accurate and stable meter for measuring volume flow. The Karman vortex flow meter, however, is not capable of performing measurement in a low flow range.